Correction of timing errors to enable long sleep times

ABSTRACT

A mobile user equipment includes a user equipment clock and a dual mode time tracker. The clock periodically wakes up the user equipment. The dual mode time tracker uses a serving cell reference signal to correct timing errors of the user equipment clock with respect to a network clock while timing errors remain minimal and otherwise uses a serving cell synchronization signal to correct timing errors of the user equipment clock. The dual mode time tracker also sets a next wakeup time as a function at least of the size of the timing errors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit from U.S. Provisional Patent ApplicationNo. 61/120,667, filed Dec. 8, 2008 which is hereby incorporated in itsentirety by reference.

FIELD OF THE INVENTION

The present invention generally relates to wireless communication, ingeneral, and in particular to power saving approaches to increasebattery life of a mobile device in a cellular network.

BACKGROUND

Cellular communication networks typically include a number of basestations, where each base station serves a “cell”, and multiple mobileunits, (referred to herein as user equipment (UEs)), that may movethrough and between the cells. The 3rd Generation Partnership Project(3GPP) is presently defining various communication standards andprotocols, including standards and protocols for the emerging nextgeneration, LTE (Long Term Evolution) Advanced cellulartelecommunication standard.

Cellular communications systems may require mobile units to communicateregularly with the base stations, including during active voice or datacalls, as well as when not actively engaged in a call. This regularcommunication utilizes battery power and may drain power from thebattery. In LTE and other standards, a UE may periodically shut downvarious receiver modules when not needed, thereby entering an “idle” or“standby” mode, to reduce its power consumption. Entering the idle orstandby mode is also called “going to sleep”.

However, even when in idle mode, the UE still needs to periodically wakeup, for example, to monitor signals that are transmitted at specifictimes, called paging slots, and to determine if it needs to fully wakeup and receive a call. The UE may also utilize paging slots to receiveadditional signals, to update its knowledge of the network, to changecell information if the UE is moving, etc. In LTE, for instance, theperiodicity of paging slots is up to 2.56 sec.

When in idle mode, a UE may turn off various components other than thosecomponents needed to maintain a timing synchronization between the UEand a network. A “slow clock unit” in the UE typically is responsiblefor waking the UE after a sleep period. It may be based on a relativelyinexpensive and power efficient crystal. However, slow clock units maybe somewhat inaccurate, which might result in the UE waking up atincorrect times. For timing correction the UE may use known signals thatthe base station periodically transmits. Normally, the UE wakes up thetime tracking module with enough time to correct any timing errorsbefore the expected receipt of the paging signal.

SUMMARY

There is provided, in accordance with an embodiment of the disclosure, amethod for waking up mobile user equipment from an idle mode. The methodincludes periodically waking up the user equipment to receive a pagingsignal. The waking up is based on a clock signal from a user equipmentclock, and may be used for example to prepare the user equipment toreceive a paging signal. A serving cell reference signal is used tocorrect timing errors of the user equipment clock with respect to anetwork clock while timing errors remain minimal and otherwise using aserving cell synchronization signal to correct timing errors of the userequipment clock.

Moreover, in accordance with an embodiment of the disclosure, the methodalso includes selecting between the serving cell synchronization signaland the serving cell reference signal as a function of a threshold levelbased on the size of the timing error corrections and a number of timeswhich the timing error corrections have remained above or below thethreshold level.

Further, in accordance with an embodiment of the disclosure, the methodincludes selecting the synchronization signal when the user equipmenttransitions from an active call state to an idle state.

Still further, in accordance with an embodiment, the method includesselecting the reference signal after a temperature of the user equipmentclock becomes stable.

Additionally, in accordance with an embodiment, setting a next wakeuptime includes scheduling the next wake up time to occur at a time thatenables receipt of a synchronization signal prior to a sub-frame of anext expected paging signal.

Alternatively, in accordance with an embodiment, setting a next wakeuptime includes scheduling the next wake up time to occur at a time thatenables receipt of a synchronization signal concurrently with a nextpaging signal and storing at least paging signals for processing afterprocessing of the synchronization signal.

There is also provided, in accordance with an embodiment, a mobile userequipment including a user equipment clock and a dual mode time tracker.The user equipment clock periodically wakes up the user equipment. Thedual mode time tracker is configured to use a serving cell referencesignal to correct timing errors of the user equipment clock with respectto a network clock while timing errors remain minimal and otherwise touse a serving cell synchronization signal to correct timing errors ofthe user equipment clock and to set a next wakeup time as a function atleast of the size of the timing errors.

Moreover, in accordance with an embodiment of the disclosure, the dualmode time tracker includes a sync time tracker, a reference time trackerand a controller. The sync time tracker uses the serving cellsynchronization signal to correct the timing errors. The reference timetracker uses the serving cell reference signal to correct the timingerrors. The controller activates one of the sync time tracker and thereference time tracker as a function of a threshold level related to amagnitude of the timing errors and a length of time they have remainedabove or below the threshold level.

Further, in accordance with an embodiment, the controller includes aunit to select the synchronization signal when the user equipmenttransitions from an active state to a non-active state.

Still further, in accordance with an embodiment, the controller includesa unit to receive a temperature of the user equipment and to select thereference signal after a temperature of the user equipment becomesstable.

Moreover, in accordance with an embodiment, the user equipment includesa scheduler to schedule the next wake up time to occur at a time thatenables receipt of a synchronization signal prior to a sub-frame of anext expected paging signal.

Additionally, in accordance with an embodiment, the user equipmentincludes a scheduler to schedule the next wakeup time to occur at a timethat enables receipt of a synchronization signal concurrently with anext paging signal and also includes a storing unit to store at leastpaging signals for processing after processing of the synchronizationsignal.

There is also provided, in accordance with an embodiment of thedisclosure, a method for waking up mobile user equipment. The methodincludes waking up a receiver module at a predetermined wakeup timeprior to receiving a periodically transmitted paging message. As afunction of the size of an expected timing error, one of a frequentlytransmitted reference signal and a less frequently transmittedsynchronization signal is selected to correct a timing error of a userequipment clock unit with respect to a network clock. A next wakeup timeis set before and close to an expected paging signal reception such thatthe reference signal can be used for time tracking if timing errorsremain minimal and otherwise, the next wake time is set to the nextexpected synchronization signal prior to an expected paging signal.

Moreover, in accordance with an embodiment, the method also includesselecting between the synchronization signal and the reference signal asa function of a threshold level of the timing correction and the numberof paging cycles that the timing errors have remained above or below thethreshold level.

Further, in accordance with an embodiment, the selecting includesselecting the synchronization signal when the user equipment transitionsfrom an active call state to an idle state.

Still further, in accordance with an embodiment, the selecting includesselecting the reference signal after a temperature of the user equipmentclock becomes stable.

Additionally, in accordance with an embodiment, setting a next wake uptime includes scheduling the next wake up time to occur at a time thatenables receipt of a synchronization signal prior to a sub-frame of anext expected paging signal.

Alternatively, in accordance with an embodiment, setting a next wakeuptime includes scheduling the next wake up time to occur at a time thatenables receipt of a synchronization signal concurrently with a nextpaging signal and storing at least paging signals for processing afterprocessing of the synchronization signal.

Finally, there is provided, in accordance with an embodiment of thedisclosure, a mobile user equipment including a user equipment wakeupunit and a dual mode time tracker. The user equipment wake up unit wakesup a receiver module at a predetermined wake up time prior to receivinga periodically transmitted paging message. The dual mode time tracker,when awake, corrects a timing error of a user equipment clock unit withrespect to a network clock with one of a frequently transmittedreference signal and a less frequently transmitted synchronizationsignal as a function of the size of expected timing errors. The wake upunit sets a next wakeup time to the next expected reference signal priorto an expected paging signal if timing errors remain minimal and to thenext expected synchronization signal prior to an expected paging signalotherwise, based on the corrected time base of the user equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure may best be understood withreference to the following figures, in which:

FIG. 1 is a block diagram illustration of a mobile device having a dualmode time tracker;

FIG. 2 is an illustration of time-frequency allocations in a frame forsynchronization signals SCH and reference signals RS in accordance withan embodiment;

FIG. 3 is a block diagram illustration of the elements of a dual modetime tracker;

FIG. 4 is a flow chart illustration of the operation of the dual modetime tracker of FIG. 3;

FIG. 5 is a timing diagram illustration of sleep times using the dualmode time tracker of FIG. 3; and

FIGS. 6A and 6B are timing diagram illustrations of alternativeembodiments.

It is noted that for simplicity and clarity of illustration, elementsshown in the figures have not necessarily been drawn to scale. Further,where considered appropriate, reference numerals are repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In various cellular communication systems, such as LTE (long termevolution), the base station is configured to regularly transmit asynchronization signal (i.e. Primary SCH and Secondary SCH referred tohere as SCH), which is conventionally employed to synchronize the UE toa cell.

The base station is also configured to regularly transmit a referencesignal (RS), also known as a pilot signal. The reference signal may betransmitted more frequently than the synchronization signal and it maybe used to determine parameters of the channel (which may change, forexample, as the user equipment UE moves through the cell). The referencesignal may also be used for time tracking.

In accordance with an embodiment, timing correction can be based onprocessing of known signals that are transmitted periodically from thebase station to the UE. Two types of known signals, at least, aretransmitted in LTE frame: reference symbols and synchronization signals.The LTE transmission frame structure generally includes 10 sub-frameseach of length 1 ms. The reference symbols, also known as pilot symbols,are located in each sub-frame at specific OFDM symbols with knownlocation over the entire frequency range in a scattered manner. Thesynchronization signals are located at specific sub-frames, i.e.sub-frame 0 and sub-frame 5. The synchronization signal has higherfrequency resolution (i.e. located in adjacent resource elements) and ina limited frequency range. Note that according to the LTE standard, theLTE frequency range can vary in between 1.4 MHz-20 MHz and thesynchronization signal can occupy only the middle 1.4 MHz frequencyband.

In accordance with an embodiment of the present disclosure, thesynchronization signal may be additionally used for time tracking withina cell, and it may also be used in conjunction with the referencesignal. A UE may select one signal over the other as a function of timecorrections and reception conditions. For example, the user equipmentmobile device may select between the synchronization signal and thereference signal as a function of a threshold level based on the size ofthe time corrections and a number of times or a number of paging cyclesduring which the time corrections have remained above or below thethreshold level. Alternatively, for example, the UE may select betweenthe synchronization signal and the reference signal as a function of anyother suitable estimation of the timing errors. As used herein, the termtime corrections refers to time corrections made by the UE as well asany other suitable estimate of timing errors in a clock of the UE withrespect to the network clock.

Reference is now made to FIG. 1, which illustrates a mobile device(called a “UE”) 10, constructed and operative in accordance with anembodiment. UE 10 may comprise a dual mode time tracker 16 and a slowclock 18. Various components that may be typically found in a UE, butare not necessary for understanding the invention, are omitted to avoidobfuscating the teaching principles of this description. Althoughspecifically described in the context of LTE, analogous principles maybe adapted to other suitable wireless and cellular communicationstandards.

Dual mode time tracker 16 may utilize the synchronization signal SCHn orthe reference signal RSn or both, to acquire and track the timing aftera sleep period and, as a result, to receive the paging signal at theproper time and to calibrate or adjust the UE timing. In accordance withan embodiment of the disclosure, selection of one of the two signals forsynchronization depends on a size of time corrections during one or moreprevious wake up periods during which UE 10 woke up. Scheduling of thenext wake up unit uses the slow clock 18 unit after timing correction.

In accordance with an embodiment, slow clock 18 is configured to wake upuser equipment mobile device at the appropriate time. In accordance withan embodiment, a next wake up time is determined based on which signal(synchronization signal or reference signal) is needed for the next timetracking operation. The dual mode time tracker 16 determines a timingcorrection and changes the time base of the UE to synchronize the UE tothe network clock. The scheduling of the next wake up period is setaccording to the corrected time base of the UE and the required wake uptiming for the next wake up, The slow clock 18 unit may update itsinternal state and configuration each wake up period to enableappropriate wake up timing.

Reference is now made to FIG. 2 which illustrates transmission timing ofsynchronization signals SCH and reference signals RS and details oftheir transmission in a LTE configuration, in accordance with anembodiment of the disclosure. FIG. 2 shows an example of timingallocations in a frame of 10 sub-frames, where each sub-frame, ortransmission time interval (TTI), is, for example, of 1 msec of length.RS signals may be transmitted at every sub-frame while synchronizationsignals SCH may be transmitted less frequently. In the example of FIG.2, they are transmitted once every 5 msec. Synchronization signals SCHare thus less available than reference signals RS for correcting timingerrors.

FIG. 2 also illustrates the transmission timing of a paging signal PCHwhich UE 10 has to be ready to receive. Paging signals PCH may betransmitted regularly. In LTE, there is a maximum interval of 2.56 secbetween transmissions of paging signals PCH.

UE 10 is configured to wake up sufficiently prior to the transmission ofa paging signal to update its timing and to calibrate its slow clock 18for the next wake up period. In accordance with an embodiment, when UE10 utilizes reference signals RS, which are transmitted at everysub-frame, UE 10 may wake up to receive the reference signal RStransmitted in several sub-frames immediately preceding the transmissionof a paging signal. In comparison, when UE 10 utilizes the periodicallytransmitted synchronization signals SCH to track the timing beforepaging reception, UE 10 may needs to wake up significantly earlier, thatis, several sub-frames prior to the transmission of a paging signal PCH,first to receive the synchronization signal and only afterward, toreceive the next paging signal PCH. In the example of FIG. 2,synchronization signals SCH are transmitted four sub-frames prior to theexpected transmission of a next paging signal. In this example, if UE 10is utilizing synchronization signal SCH for time tracking, UE 10 maywake up to receive its synchronization signal SCH, and then wait severalsub-frames, during the interim, in an awake mode to monitor receipt of apaging signal PCH that may be transmitted during the next paging slot.Thus, in a worst case, the UE 10 might need to wake up severalsub-frames prior to transmission of the paging signal PCH. For example,there can be a specific configuration where the synchronization signalSCH and paging channel are in the same sub-frame. The UE 10 can firstuse synchronization signal SCH for time tracking and then decode thepaging channel. In a case where the UE 10 wakes up for synchronizationsignal SCH for time tracking several sub-frames before paging receptionof paging signal PCH, it can turn into a partial sleep period until thereception of the paging signal PCH.

Irrespective of which signal is being used for time-tracking, if the UEneeds to enter idle mode until the next paging period the UE 10 maycalibrate the slow clock 18 according to the correct time base and thenreturn to idle mode.

FIG. 2 also illustrates the transmission patterns of reference signalsRS and synchronization signals SCH for two sub-frames, or TTIs. Eachsub-frame comprises a plurality of time-frequency resource blocks, eachcovering interval of time and frequency resource elements. The exampleof FIG. 2, presents time frequency “map” that include 14 time intervals(called OFDM symbols) per sub-frame and 24 resource elements per timeinterval which represents only part of the frequency range. It should benoted here that the actual number of OFDM symbols and number of usefulresource elements is configurable in LTE.

The reference signals RS are transmitted only in very specific OFDMsymbols spread over the frequency band. In contrast, synchronizationsignals SCH are transmitted in a block of adjacent resource elementsover a limited frequency band, typically within the middle of thefrequency range of the useful frequency band. In accordance with LTEstandards, the reference signals RS are spread across 1.4 MHz-20 Mzwhile the synchronization signal may occupy a 1.4 MHz frequency band inthe middle of the frequency range.

Since the synchronization signals SCH are transmitted over anuninterrupted frequency band, the synchronization signals SCH may have ahigher frequency resolution. As a result, time tracking withsynchronization signals SCH may be able to track higher timing errorsthan reference signals RS, which may exhibit a timing ambiguity(typically about +/−10 μsec, in LTE) and thus, reference signals RS maybe useful for time tracking only of relatively small timing errors.However, reference signals RS are transmitted more frequently and thusare available close to those sub-frames in which a paging signal PCH maybe transmitted.

Reference is now made to FIG. 3, which illustrates elements of dual modetime tracker 16. In accordance with an embodiment, tracker 16 comprisesa sync time tracker 32 (operating on a current synchronization signalSCHn), a reference time tracker 34 (operating on a current referencesignal RSn) and a controller 30 to activate one of the time trackers 32,34. Controller 30 may selectively activate sync time tracker 32 andreference time tracker 34 to change the sleep time of UE 10 with littleor no impact on the decoding performance of UE 10. When relatively largesleep time errors are expected, controller 30 may activate sync timetracker 32; conversely, when relatively small sleep time errors areexpected, controller 30 may activate reference time tracker 34.

Sync time tracker 32 may be embodied in any suitable element which istypically found in user equipment 10 and which operates onsynchronization signal SCHn. In accordance with an embodiment, thesuitable elements are configured to process the synchronization signalto, among other things, determine the timing. For example, a neighborcell searcher processes synchronization signals to determine cell timingand to select a new cell, as necessary. Thus, a suitably configuredneighbor cell searcher comprises elements capable of determining timingfrom a synchronization signal, particularly if, as in this embodiment,the cell is already identified.

In an embodiment, reference time tracker 34 is any suitable elementwhich is typically found in user equipment 10 and which operates onreference signals RSn, also called “pilot signals”. All such elementstypically determine the timing of the reference signal in addition totheir other calculations, such as channel estimation.

For example, in accordance with an embodiment and as shown in theflowchart of FIG. 4, to which reference is now made, when in an initialstate, such as after start up, controller 30 may activate sync timetracker 32. Sync time tracker 32 may use current synchronization signalSCHn to determine the current timing, and to correct the UE timing forpaging reception.

Whenever timing corrections are large, sync time tracker 32 is inoperation. However, once the timing corrections are small, such as lessthen a predefined value such as 3 μs, and the timing corrections remainsmall for N1 paging periods, then controller 30 may activate referencetime tracker 34. Reference time tracker 34 may remain activated for aslong as it can suitably track the timing. However, if, for N2 pagingsignals, reference time tracker 34 fails to suitably track the timing,controller 30 may revert back to making timing corrections with synctime tracker 32. In accordance with an embodiment, N1 may be 5-6 and N2may be 1, for example.

In accordance with an embodiment, reference time tracker 34 successfullytracks the timing if time tracking corrections are small, such as lessthan 2 μsec, and if the signal-to-noise ratio (SNR) exceeds somethreshold, for example −5 dB

After timing corrections it is possible to configure the slow clock 18,for the next wakeup time. Controller 30 may also provide an indicationto the slow clock unit of which time tracker 32 or 34 is currentlyselected, such that slow clock 18 may use this information to configureits parameters to determine next wakeup time of the UE.

Reference is now made to FIG. 5, which comparatively illustrates thesleep time of UE 10, in accordance an embodiment. FIG. 5 shows amultiplicity of sub-frames, where each set of sub-frames 0 through 9forms a frame. Paging signal PCH is transmitted at sub-frame 4 of eachframe. FIG. 5 shows only a select few sub-frames, with a gap indicatedby wiggling lines.

As mentioned hereinabove, UE 10 may “go to sleep” after waking up toreceive a paging signal. If sync tracker 32 is currently active, slowtime clock 18 may be configured to wake up UE 10 from a short sleepperiod 40, in order to receive synchronization signal SCH (which may beexpected, in the example of FIG. 5, at sub-frame 0). During short sleepperiod 40, UE 10 may turn off the receiver and various processors,except modules that may be necessary to determine when to next wake up.

However, in accordance with LTE standards and as seen in the example ofFIG. 5, synchronization signal SCH may be transmitted in sub-frame 0while paging signal PCH may be transmitted in sub-frame 4. Thus, UE 10may wake up 4 ms before the paging signal PCH is transmitted. Synctracker 32 may wake up for a wake period 42 during which it may processsynchronization signal SCH to track the timing errors, such as thoseerrors that are due to sleep time and/or propagation variation, afterwhich may follow a partial sleep period 44 until paging signal PCH isexpected, during which some of the modules of UE 10, may shut down.

If, instead, controller 30 activates reference tracker 34 for makingtiming corrections, in accordance with an embodiment slow time clock 18will be configured to wake up UE 10 from a long sleep period 46 at thesub-frame before the next expected paging signal, in order to receiveand process reference signal RS before receiving paging signal PCH.Thus, in the example of FIG. 5, long sleep period 46 ends at sub-frame 3of the second frame and paging signal PCH begins at sub-frame 4. As seenin FIG. 5, long sleep period 46 is somewhat longer than short sleepperiod 40 and may avoid the need for partial sleep period 44, thuspotentially reducing energy consumption over time. It should be notedthat this is just an example, and in general the UE may wakeup severalsub-frames before paging reception also using the reference symboltracking approach.

In accordance with an embodiment, controller 30 may utilize otherinformation to determine when to switch between trackers 32 and 34. Forexample, among the main contributors to sleep time errors areimperfections in the slow clock crystals and variations in thepropagation conditions. The slow clock crystal suffers from frequencydrift due to temperature variations. The temperature may change due to achange in the internal and/or external conditions of the UE, forexample, the UE may have changed location from a cold location to a warmlocation. Alternatively, the UE may have stopped transmitting; this mayalso cause the temperature of the processor circuitry to cool down.

In accordance with an embodiment, controller 30 may activate sync timetracker 32 at any time whenever temperature variability may be expected,such as for a pre-determined time after the power amplifier is turnedoff, or until a temperature detector indicates that the temperature atthe slow clock crystal has attained suitable stability. Once suitabletemperature stability has been reached, in accordance with anembodiment, controller 30 activates reference time tracker 34.

Reference is now made to FIGS. 6A and 6B, which illustrate alternativeembodiments. Like FIG. 5, FIGS. 6A and 6B show a multiplicity of labeledsub-frames, where each set of sub-frames 0 through 9 forms a frame. Asbefore, paging signal PCH is transmitted at sub-frame 4 of each frame.

In the embodiments of FIGS. 6A and 6B, controller 30 may wake up UE 10for a synchronization signal SCH in the sub-frames close to thesub-frame of a paging signal PCH. Currently, in accordance with the LTEstandard, for example, paging information may be transmitted on specificsub-frames. In accordance with current LTE standards, sub-frames inwhich a PCH is transmitted may be either the same sub-frame as a SCH, asshown in FIG. 6B, or one sub-frame prior to the SCH signal, as shown inFIG. 6A. In order to fully process synchronization signal SCH beforeprocessing paging signal PCH, in this embodiment, controller 30 mayindicate to UE 10 to record the input samples of paging signal PCH forlater processing. In accordance with an embodiment of the disclosure,since the timing errors may not yet have been corrected and thus, theexact timing not yet known, the recordation of paging signals PCH maybegin before the expected timing of paging signal PCH and may endafterward.

Controller 30 activates sync time tracker 32 to detect the correcttiming. UE 10 then utilizes the corrected timing whileprocessing/decoding offline any received paging signal PCH. Thismethodology facilitates longer sleep times, as it eliminates or reducesthe need to wakeup UE 10 significantly before the sub-frame of pagingsignal PCH in order to receive synchronization signal SCH.

In both FIGS. 6A and 6B, offline paging processing, labeled 70, occursafter the processing activity, labeled 71, of sync time tracker 32.Controller 30 may then indicate a long sleep time 72 once offline pagingprocessing 70 may finish, assuming that paging signal PCH was not topage UE 10. This approach requires additional buffering in order torecord at least some of the paging samples.

Unless specifically stated otherwise, as apparent from the disclosureherein, any “processing,” “computing,” “calculating,” “determining,” orsimilar operations, refer to operations that may be performed indedicated computing hardware, in a generalized computer device usingfirmware or software.

While certain features of embodiments of the invention have beenillustrated and described herein by way of example, many modifications,substitutions, variations, changes, combinations and equivalents may beapparent to those of ordinary skill in the art. Accordingly, the scopeof the present invention as embodied in the claims appended hereto isintended to cover all such modifications, substitutions, variations,changes, combinations and equivalents occurring to a person of ordinaryskill based on the foregoing description and which are not disclosed inthe prior art.

1. A method for waking up mobile user equipment from an idle mode, themethod comprising: periodically waking up said user equipment to receivea paging signal; and using a serving cell reference signal to correcttiming errors of said user equipment clock with respect to a networkclock while timing errors remain minimal and otherwise using a servingcell synchronization signal to correct timing errors of said userequipment clock.
 2. The method according to claim 1, further comprising:selecting between said serving cell synchronization signal and saidserving cell reference signal as a function of a threshold level basedon the size of the timing error corrections and a number of times duringwhich said timing error corrections have remained above or below saidthreshold level.
 3. The method according to claim 1, further comprisingselecting said synchronization signal when said user equipmenttransitions from an active call state to an idle state.
 4. The methodaccording to claim 1, further comprising using said synchronizationsignal while a temperature of the user equipment is not stable.
 5. Themethod according to claim 1, wherein setting a next wakeup timecomprises scheduling said next wake up time to occur at a time thatenables receipt of a synchronization signal prior to a sub-frame of anext expected paging signal.
 6. The method according to claim 1, whereinsetting a next wakeup time comprises scheduling said next wake up timeto occur at a time that enables receipt of a synchronization signalconcurrently with a next paging signal and storing at least pagingsignals for processing after processing of the synchronization signal.7. A mobile user equipment comprising: a user equipment clock toperiodically wake up said user equipment to receive paging signals; anda dual mode time tracker configured (i) to use a serving cell referencesignal to correct timing errors of said user equipment clock withrespect to a network clock while timing errors remain minimal andotherwise to use a serving cell synchronization signal to correct timingerrors of said user equipment clock, and (ii) to set a next wakeup timeas a function at least of the size of said timing errors.
 8. The deviceaccording to claim 7, wherein said dual mode time tracker comprises: async time tracker configured to use said serving cell synchronizationsignal to correct said timing errors; a reference time trackerconfigured to use said serving cell reference signal to correct saidtiming errors; and a controller configured to activate one of said synctime tracker and said reference time tracker as a function of athreshold level related to a magnitude of the timing corrections and alength of time they have remained above or below said threshold level.9. The device according to claim 8, wherein said controller comprises aunit to select said synchronization signal when said user equipmenttransitions from an active state to a non-active state.
 10. The deviceaccording to claim 9, wherein said controller comprises a unit toreceive a temperature of said user equipment and to select saidreference signal after a temperature of said user equipment becomesstable.
 11. The device according to claim 7, further comprising ascheduler to schedule said next wake up time to occur at a time thatenables receipt of a synchronization signal prior to a sub-frame of anext expected paging signal.
 12. The device according to claim 7,further comprising a scheduler to schedule said next wakeup time tooccur at a time that enables receipt of a synchronization signalconcurrently with a next paging signal and further comprises a storingunit to store at least paging signals for processing after processing ofthe synchronization signal.
 13. A method for waking up mobile userequipment, comprising: waking up a receiver module of the user equipmentat a predetermined wakeup time prior to receiving a periodicallytransmitted paging message; as a function of the size of an expectedtiming error, selecting one of a frequently transmitted reference signaland a less frequently transmitted synchronization signal to correct atiming error of the user equipment clock unit with respect to a networkclock; and setting a next wakeup time (i) before and close to anexpected paging signal reception such that the reference signal can beused for time tracking if timing errors remain minimal and (ii) to thenext expected synchronization signal prior to an expected paging signalotherwise.
 14. The method according to claim 13, further comprising:selecting between said synchronization signal and said serving cellreference signal as a function of a threshold level based on the size ofthe timing error corrections and a number of paging cycles that saidtiming error corrections have remained above or below said thresholdlevel.
 15. The method according to claim 13, wherein said selectingcomprises selecting said synchronization signal when said user equipmenttransitions from an active call state to an idle state.
 16. The methodaccording to claim 13, wherein said selecting comprises selecting saidreference signal after a temperature of said user equipment clockbecomes stable.
 17. The method according to claim 13, wherein setting anext wakeup time comprises scheduling said next wake up time to occur ata time that enables receipt of a synchronization signal prior to asub-frame of a next expected paging signal.
 18. The method according toclaim 13, wherein setting a next wakeup time comprises scheduling saidnext wake up time to occur at a time that enables receipt of asynchronization signal concurrently with a next paging signal andstoring at least paging signals for processing after processing of thesynchronization signal.